Stereochemistry of Alkanes and Cycloalkanes | CHEM 2010, Study notes of Organic Chemistry

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Stereochemistry of Alkanes and Cycloalkanes Based on McMurry’s Organic Chemistry, 6th edition, Chapter 4 2 The Shapes of Molecules  The three-dimensional shapes of molecules result from many forces  A molecule may assume different shapes, called conformations, that are in equilibrium at room temperature (the conformational isomers are called conformers, emphasis on the first syllable)  The systematic study of the shapes molecules and properties from these shapes is stereochemistry  The field of stereochemistry is one of the central parts of organic chemistry and includes many important topics 5 Conformations of Ethane staggered conformation eclipsed conformation 6 Representing Conformations  Sawhorse representations show molecules at an angle, showing a molecular model  C-C bonds are at an angle to the edge of the page and all C-H bonds are shown  Newman projections show how the C-C bond would project end-on onto the paper  Bonds to front carbon are lines going to the center  Bonds to rear carbon are lines going to the edge of the circle Newman Projections Back carbon Hy Newman projection 10 Ethane’s Conformations 0o 60o 120o 180o 240o 300o 360(0)o Ethane’s Conformations Eclipsed conformers 12 kJ/mol ie H H H H H H H H H H Ey ee Be ge ae x le et l | | | | | o° 60° 120° 180° 240° 300° 360° © 2004 Thomson/Brooks Cole: 11 12 4.2 Conformations of Propane  Propane (C3H8) torsional barrier around the carbon– carbon bonds 14 kJ/mol  Eclipsed conformer of propane has two ethane-type H–H interactions and an interaction between C–H and C–C bond Staggered Conformations of Butane oa Anti Gauche Least stable eclipsed 15 16 Conformations of Butane Conformations of Butane i6 19 kJ/mol a vial cH, CH ae HS A a A, & os at Anti Anti U i L j | 180° 120° 7 or “= 120° 180° Dihedral angle between methyl groups Sources of strain caused by rotation: TABLE 4.1 Energy Costs for Interactions in Alkane Conformers Energy cost Interaction Cause (kJ/mol) (kcal/mol) H <> Heclipsed Torsional strain 4.0 1.0 H <> CH; eclipsed Mostly torsional strain 6.0 1.4 CH; <> CHs eclipsed Torsional plus steric strain 11 2.6 CH; <> CHz3 gauche Steric strain 3.8 0.9 ©2004 Thomson - Brooks/Cole 20 1-chloropropane Cl H H H H CH; Most stable (staggered) ‘©2004 Thomson - Brooks/Cole- H,0C1 A, Least stable (eclipsed) 21 Hydrocarbon Chains: Staggered VON ONY ONY ONG Ze, Ze, ZEN, ZN, Zon, UH cog eg “» “s “es “1 “’ H HH HH HH HH H fl © 2004 Thomson/Brooks Cale 22 Heats of Combustion 3 (CH,), + —- 0, —— nCO,+nH,0 + Heat ©2004 Thomson - Brooks. Stability of Cycloalkanes 2 5 Strain energy (kJ/mol) wese = | 3 4 5 6 7 & 9 10 11 12 13 14 Ring size 27 4.5 The Nature of Ring Strain  Rings larger than 3 atoms are not flat (planar).  Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering  Larger rings have many more possible conformations than smaller rings and are more difficult to analyze 30 Steric Strain Strain Energies TABLE 4.1 Energy Costs for Interactions in Alkane Conformers Energy cost Interaction Cause (kJ/mol) (kcal/mol) H <> Heclipsed Torsional strain 4.0 LO H <> CH; eclipsed Mostly torsional strain 6.0 14 CH; <> CHs eclipsed Torsional plus steric strain al 2.6 CH; <> CHs gauche Steric strain 3.8 0.9 ©2004 Thomson - Brooks/Cole 31 32 Summary: Types of Strain Angle strain - expansion or compression of bond angles away from most stable Torsional strain - eclipsing of bonds on neighboring atoms Steric strain - repulsive interactions between nonbonded atoms in close proximity 35 “Bent” bonds in cyclopropane: less than maximum orbital overlap 36 4.7 Conformations of Cyclobutane and Cyclopentane  Cyclobutane has less angle strain than cyclopropane but more torsional strain because of its larger number of ring hydrogens  Cyclobutane is slightly bent out of plane - one carbon atom is about 25° above  The bend increases angle strain but decreases torsional strain Cyclobutane e 2 (c) Not quite ale eclips en H * ze ew Sn Not quite eclipsed © 2004 Thomson/Brooks Cole 37 40 4.8 Conformations of Cyclohexane  Substituted cyclohexanes occur widely in nature  The cyclohexane ring is free of angle strain and torsional strain  The conformation has alternating atoms roughly in a common plane, and tetrahedral angles between all carbons  This is called a chair conformation 41 Chair Conformations Cholesterol: three chair conformations HO Cholesterol ©2004 Thomson - Brooks/Cole 42 Axial and Equatorial Bonds Ring axis --#f Ring equator 46 Axial and Equatorial Positions  Each carbon atom in cyclohexane has one axial and one equatorial hydrogen  Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement Drawing the Axial and Equatorial Hydrogens Axial bonds: The six axial honds, one on each carbon, — are parallel and alternate up-down. Equatorial bonds: The six equatorial bonds, one on J | ~7 | - each carbon, come in three [- _—— L- sets of two parallel lines. Each set is also parallel to two ring bonds, Equatorial bonds alter- nate between sides around the ring, Completed cyclohexane 47 Conformational Mobility Move this carbon down } Move this - carbon up || Ring-flip || Ring-flip . . 51 Bromocyclohexane  When bromocyclohexane ring-flips the bromine’s position goes from equatorial to axial and so on  At room temperature the ring-flip is very fast and the structure is seen as the weighted average Bromocyclohexane Axial bromocyclohexane Equatorial bromocyclohexane Thomson - Brooks Cole 52 55 Energy and Equilibrium  The relative amounts of the two conformers depend on their difference in energy E = RT ln K  R is the gas constant [8.315 J/(K•mol)], T is the Kelvin temperature, and K is the equilibrium constant between isomers 56 1,3-Diaxial Interactions  Difference between axial and equatorial conformers is due to steric strain caused by 1,3-diaxial interactions  Hydrogen atoms of the axial methyl group on C1 are too close to the axial hydrogens three carbons away on C3 and C5, resulting in 7.6 kJ/mol of steric strain 1,3-Diaxial Interactions Steric interference He--<-.CH3 aL. a Ne i Women rs 5 3 Monosubstituted Cyclohexanes TaBLe 4.2 Steric Strain in Monosubstituted Cyclohexanes qe Strain of one H-Y 1,3-diaxial interaction Y (Ij /mol) (kcal/mol) —F 0.5 0.12 —Cl 1.0 0.25 —Br 1.0 0.25 —OH 21 0.5 —CH3 3.8 0.9 —CH2CH3 4.0 0.95 —CH(CH3). 46 11 —C(CHa)s3 11.4 27 —CeH; 6.3 15 —CO.H 2.9 0.7 —CN 0.4 0.1 ©2004 Thomson - Brooks/Cole 61 4.12 Conformational Analysis of Disubstituted Cyclohexanes  In disubstituted cyclohexanes the steric effects of both substituents must be taken into account in both conformations  There are two isomers of 1,2- dimethylcyclohexane. cis and trans 62 4.12 Conformational Analysis of Disubstituted Cyclohexanes  In the cis isomer, both methyl groups same face of the ring, and compound can exist in two chair conformations  Consider the sum of all interactions  In cis-1,2, both conformations are equal in energy 65 Trans-1,2- Dimethylcyclohexane  Methyl groups are on opposite faces of the ring  One trans conformation has both methyl groups equatorial and only a gauche butane interaction between methyls (3.8 kJ/mol) and no 1,3-diaxial interactions  The ring-flipped conformation has both methyl groups axial with four 1,3-diaxial interactions 66 Trans-1,2- Dimethylcyclohexane  Steric strain of 4  3.8 kJ/mol = 15.2 kJ/mol makes the diaxial conformation 11.4 kJ/mol less favorable than the diequatorial conformation  trans-1,2- dimethylcyclohexane will exist almost exclusively (>99%) in the diequatorial conformation Trans-1,2- Dimethylcyclohexane trans-1,2-Dimethyicvclohexane One gauche interaction (3.8 kJ/mol) H32 7 am | Ring-flip t-Butyl Groups H <--------2,Cl H* H i ai H,C™ / HC 2 x 1.0 = 2.0 kJ/mol steric strain ©2004 Thomson - Brooks/Cole H3C_ CH, . H,c—Cc~ \"“H H ‘H H Cl 2x 11.4 = 22.8 kJ/mol steric strain 70 t-Butyl Groups Br cis-1-Bromo-4-fert-butylcyclohexane (axial bromine) ©2004 Thomson - Brooks/Cole Br 71 t-Butyl Groups _Br Br trans-1-Bromo-4-tert-butylcyclohexane (equatorial bromine) 72 75 Problem 4.39: Galactose has an axial OH group at C4. Draw the chair: 76 Solution: O OH OH HO HO CH2OH Galactose 77 4.13 Boat Cyclohexane  Cyclohexane flips through a boat conformation  Less stable than chair cyclohexane due to steric and torsional strain  C-2, 3, 5, 6 are in a plane  H on C-1 and C-4 approach each other closely enough to produce considerable steric strain  Four eclipsed H-pairs on C- 2, 3, 5, 6 produce torsional strain  ~29 kJ/mol (7.0 kcal/mol) less stable than chair 80 4.14 Conformations of Polycyclic Molecules  Decalin consists of two cyclohexane rings joined to share two carbon atoms (the bridgehead carbons, C1 and C6) and a common bond Decalin i cH,| CH, 2 10 2 H, c Ts on “i, OH, . ; HC. Uo Nn _ CHe 8 i. 4 CH, | CH, 7 5 H Decalin (two fused cyclohexane rings) ©2004 Thomson - Brooks/Cole 81 82 4.14 Conformations of Polycyclic Molecules  Two isomeric forms of decalin: trans fused or cis fused  In cis-decalin hydrogen atoms at the bridgehead carbons are on the same face of the rings  In trans-decalin, the bridgehead hydrogens are on opposite faces  Both compounds can be represented using chair cyclohexane conformations  Flips and rotations do not interconvert cis and trans Cholesterol 5 apie ©2004 Thomson - Brooks/Cole ‘Testosterone CH, Testosterone (a steroid) ©2004 Thomson - Brooks/Cole CH. 3 OH 86 Bicyclic Compounds A 1-carbon bridge A 2-carbon bridge —— , Zs Bridgehead carbons Norbornane (Bicyclo[2.2.1]heptane) ©2004 Thomson - Brooks/Cole 87